Why am I unable to use TEntity?

Sometimes it feels like even the simplest things are hard to understand. The type TEntity is often used in code references and is utilized for general entity handling in entity data models. Why does it say "Unknown type 'TEntity'" when I try to use it in my code? What does "Unknown kind" mean? Does this just work with.net 4.0?

Using.net 3.5, btw.

I'm attempting to utilize the following code:

public TEntity ExecuteFirstorDefault<TEntity>(ObjectQuery<TEntity> objectQuery)
  {
    try
    {
      return objectQuery.FirstOrDefault();
    }
    catch (EntitySqlException ex)
    {
      throw ex; //TODO: Replace with handling code
      //additional exceptions as described in Chapter 18
    }
  }